B American Society for Mass Spectrometry, 2018 J. Am. Soc. Mass Spectrom. (2018) DOI: 10.1007/s13361-018-2007-x RESEARCH ARTICLE Optimized Electrostatic Linear Ion Trap for Charge Detection Mass Spectrometry Joanna A. Hogan, Martin F. Jarrold Chemistry Department, Indiana University, Bloomington, IN 47405, USA Abstract. In charge detection mass spectrometry (CDMS), ions are passed through a detection tube and the m/z ratio and charge are determined for each ion. The uncertainty in the charge and m/z determinations can be dramatically reduced byembeddingthedetectiontubeinanelectro- static linear ion trap (ELIT) so that ions oscillate back and forth through the detection tube. The resulting time domain signal can be analyzed by fast Fourier transforms (FFTs). The ion’s m/z is proportional to the square of the oscillation frequency, and its charge is derived from the FFT magnitude. The ion oscillation frequency is dependent on the physical dimensions of the trap as well as the ion energy. A new ELIT has been designed for CDMS using the central particle method. In the new design, the kinetic energy depen- dence of the ion oscillation frequency is reduced by an order of magnitude. An order of magnitude reduction in energy dependence should have led to an order of magnitude reduction in the uncertainty of the m/z determi- nation. In practice, a factor of four improvements was achieved. This discrepancy is probably mainly due to the trajectory dependence of the ion oscillation frequency. The new ELIT design uses a duty cycle of 50%. We show that a 50% duty cycle produces the lowest uncertainty in the charge determination. This is due to the absence of even-numbered harmonics in the FFT, which in turn leads to an increase in the magnitude of the peak at the fundamental frequency. Keywords: Ion trap, Electrostatic linear ion trap, ELIT, Charge detection mass spectrometry, CDMS Received: 17 January 2018/Revised: 25 May 2018/Accepted: 29 May 2018 Introduction The solution to this problem invariably involves a single particle approach where the mass is determined directly for lectrospray has extended mass spectrometry to the anal- individual ions [5]. The nanomechanical oscillator is one such E ysis of native, noncovalent protein complexes and other approach. Here the frequency shift that occurs when a single macromolecular structures [1, 2]. Time-of-flight mass spec- molecule or ion adsorbs on a nanoscale oscillator is used to trometry (TOFMS) has long been the primary MS technique deduce its mass [6–8]. Charge shifting is another approach that for the analysis of large ions because in theory there is no upper is closer to traditional mass spectrometry. Here the m/z is mass limit. However, the upper mass attainable by TOFMS is measured for a single ion, the charge is shifted, and the m/z limited by the low detection efficiency of commonly used re-measured [9–14]. This method has been used most widely detectors for high m/z ions [3]. In addition, as analyte mass with a quadrupole ion trap (QIT) and optical detection. While increases, so does heterogeneity. Many large biological com- single particle QIT-MS has no upper mass limit and excellent plexes are inherently heterogeneous, and adduct formation m/z and charge accuracy, measurements can take a minute or becomes more prevalent for larger ions [4]. Heterogeneity more per ion, and particles smaller than around 50 nm require leads to broadened m/z peaks and unresolvable charge states. special techniques. The method is better suited to monitoring Without charge state resolution, it is impossible to determine single ions for long times rather than recording mass distribu- the mass. tions [12, 15–18]. Finally, there are methods based on measuring the m/z and Correspondence to: Martin Jarrold; e-mail: [email protected] charge of individual ions. This has been done with Fourier J. A. Hogan, M. Jarrold: Linear Ion Trap for Charge Detection Mass Spectrometry transform ion cyclotron resonance [19] where the charge is those used in lasers and interferometers. The first ELIT was determined from the charge induced on the detector plates. described by Zajfman and coworkers as an alternative to However, because the ion is not surrounded by the detector heavy-ion storage rings for investigation of high energy ion plates, the induced charge depends on the ion’strajectoryand beams [44]. In addition to CDMS where single ions are so the charge cannot be determined accurately. A quadrupole trapped, ELITs have been used to perform mass spectrometry ion trap coupled to an external charge detecting plate has also with packets of ions [45]. McLuckey and coworkers have been used to measure the m/z and charge of single ions [20, 21]. expanded the functionality of ELITs with packets of ions, Here again, the charge cannot be determined accurately. demonstrating both collision-induced dissociation and surface TOFMS with cryogenic detectors is another technique in this induced dissociation MS/MS [46, 47]. They have also mea- class [22–27]. However, the detector response is not linearly sured collisional cross sections for m/z selected ion packets by proportional to the ion charge above a few charges, so charge measuring the transient signal decay after the addition of gas to determination is not currently possible for large ions with the ELIT [48]. hundreds of charges. Last but not least is charge detection mass In this manuscript, we describe the factors that need to be spectrometry (CDMS). Here the ion is passed through a considered to optimize the performance of an ELIT for CDMS conducting cylinder and the induced charge is detected by a measurements. A new ELIT design, optimized for CDMS charge sensitive amplifier. The time of flight through the cyl- measurements, is described. The performance of the new ELIT inder is used (along with the ion energy) to deduce the m/z, and is evaluated, and it is shown to have a significantly improved the amplitude of the signal is proportional to the charge. If the m/z resolving power over that achieved previously. cylinder is long enough, the amplitude is independent of the ion’s trajectory through the cylinder. Masses into the teradalton regime have been measured [28]. ELIT Design Considerations The first use of CDMS was in the 1960s when Shelton and In CDMS, the uncertainty in the mass determination depends coworkers used single-pass CDMS to determine the masses of on the uncertainties in the charge and m/z measurements. fast-moving, micron-sized iron particles [29]. In 1995, Historically, the main issues with CDMS have been the large Fuerstenau and Benner coupled an electrospray source to a uncertainty in the charge measurement and the high LOD. The single-pass CDMS detector and used it to measure the mass ELIT employed in our prototype CDMS instrument [49]was of DNA ions [30]. The uncertainty in the charge was ± 75 e based on the Bconetrap^ design of Cederquist and coworkers (elementary charges) which led to a large uncertainty in the [50]. However, rather than measure the TOF of the ions derived mass. The large uncertainty in the charge prompted through the detector, the time domain signal from the trapped Benner to employ an electrostatic linear ion trap (ELIT), where ion is stored and analyzed by fast Fourier transforms (FFTs). the ions oscillate back and forth through the detector cylinder, The use of FFTs enables the detection of charges which do not to perform multiple measurements for each ion. The uncertain- rise above the noise in the time domain and lowers the LOD to ty in the charge decreases with n1/2,wheren is the number of <7e[51]. The m/z is inversely proportional to the square of the measurements [31]. Benner reported a theoretical uncertainty fundamental frequency of ± 2.4 e. However, to reduce the number of false positives, the limit of detection (LOD) (the smallest charge that can be C m=z ¼ ð1Þ reliably detected) was placed at around five times the root mean f 2 square deviation (RMSD) of the noise. The high LOD in these measurements (250 e) limited the applicability to very large where C is a constant that is a function of the ion energy as well ions. as the dimensions of the trap. It is determined from ion trajec- More recently, interest in CDMS has been rekindled as there tory simulations. The charge is proportional to the FFT mag- has been an increasing desire to perform mass measurements nitude (when the number of ion cycles is taken into account). beyond the range of conventional mass spectrometry [28, 32– The uncertainty in the charge depends on the trapping time 43]. Antoine, Dugourd, and coworkers have used single-pass which can be extended by lowering the pressure in the trap CDMS to characterize nanoparticles, synthetic polymers and [51]. With a trapping time of 3 s, the charge uncertainty was amyloid fibrils [33–35]. Williams and coworkers have recently reduced to 0.2 e (RMSD) where it is possible to assign the shown that ELIT CDMS can be used to measure collision cross charge state with almost perfect accuracy [52]. Thus, the un- sections [37, 38]. Our group has mainly used CDMS to analyze certainty in m/z is now the limiting factor in obtaining high viruses and virus capsids, including the detection of assembly precision mass measurements. One of the factors contributing intermediates for hepatitis B virus capsids [39, 40], the charac- to the uncertainty in the m/z measurement is the dependence of terization of bacteriophage P22 [41, 42], and the resolution of the oscillation frequency on the ions’ kinetic energy. A dual fully and partially packed adeno-associated virus, a gene ther- hemispherical deflection analyzer (HDA) is used to limit the apy vector [43].